Actual sugar-alcohols produced industrially include sorbitol, mannitol, maltitol, xylitol and others and they are used in quantity as food additives, industrial materials or pharmaceutical materials.
In general, these sugar-alcohols are produced by the heating reaction of sugars under the hydrogen pressure in the presence of a hydrogenation catalyst.
A supported ruthenium catalyst, a Raney catalyst or the like are known as the hydrogenation catalyst used for sugar-alcohol production.
Though the supported ruthenium catalyst presents a very high catalytic activity, it provokes inconveniently sugar isomerization, decomposition and polymerization during the hydrogenation.
For solving this inconvenience, the British patent No. 867,689 describes Ru—Pd catalyst carried by activated carbon wherein ruthenium is added to palladium, however, the purity of sorbitol produced by the patent remained between 93.5 and 97.5%.
On the other hand, the Japanese TOKKYO-KOKAI-KOHO (18-month Publication of Unexamined Patent Application) SHOWA (hereinafter referred to as TOKKAISHO) 51-4370 describes Ru catalyst carried by zeolite aluminosilicate while the Japanese TOKKAISHO 51-82208 describes an example of glucose hydrogenation by Ru catalyst carried by crystalline clay aluminosilicate, but their results were not satisfactory because the purity of sorbitol is not superior to 99% in either case.
Raney catalyst is a catalyst activated by dissolving a part of metals such as aluminum, zinc and silicon by an alkali from alloys of catalytic metals such as nickel, copper and iron and metals such as aluminum, zinc and silicon.
The catalyst presents in general a low catalytic activity and a high catalyst deterioration and the catalyst cost assumes disadvantageously a great part of the product price.
Moreover, as the catalyst is supplied mainly in powder form and used by batch method, it is necessary to provide a step for separating the catalyst from the reaction solution after the hydrogenation, the production cost increases as much.
In order to remedy this defect, a variety of Raney catalysts for fixed bed have been developed, none of them leading to a satisfactory solution.
For instance, the Japanese TOKKAISHO 50-099987 describes a method for producing a Raney catalyst for fixed bed based on nickel, cobalt or copper precipitation type catalyst.
In the method, conventional nickel, cobalt or copper precipitation type catalyst is blended and formed with powder metal/aluminum alloy and then treated at a high temperature using steam. During this step, γ-Al2O3 acting as binder is generated, but as the formed body is destroyed by the dissolution of γ-Al2O3 in the step of activation by an alkali, it is not appropriate for the production of Raney catalyst for fixed bed.
The Japanese TOKKAISHO 47-27888 describes a method for producing a Raney catalyst for fixed bed by dropping melted alloy in or on a chilled solvent to form a catalyst and activating it.
It is important to increase the density of catalyst to be loaded and to regulate the solution flow in the fixed bed for efficient hydrogenation during the sugar-alcohol production. For this sake, it is preferable to limit the catalyst grain diameter to 4 mm or less but, if the grain diameter is too small, the resistance increases and the solution flow slows down in a way to provoke obstruction of broken catalyst, so the preferable grain diameter is 2 to 4 mm approximately.
However, if it is desirable to obtain an uniform granulating in the catalyst production, the grain diameter of this range will only be obtained in the sacrifice of the yield by the method described in the Japanese TOKKAISHO 47-27888 wherein alloy grains are produced by dropping melted metal from an orifice. Additionally, alloy grains out of this diameter range should be melted again so as to increase the cost as much.
Theretofore, the object of the present invention is to obtain a Raney catalyst for fixed bed remedying various problems mentioned hereinbefore and to produce a high purity sugar-alcohol at a low cost using the same.